main: Ignore some modifiers when matching events to keybindings

Some modifiers like NumLock or ScrollLock don't make sense in
keybindings, which is why we ignore them in mutter when matching
events to keybindings; for keybindings in the overview, we do
the matching ourselves, so filter the same modifiers as mutter.

https://bugzilla.gnome.org/show_bug.cgi?id=665215